Invasive Haemophilus influenzae Infections: 2025 Trends and Vaccine Efficacy

The epidemiological landscape of invasive Haemophilus influenzae disease has undergone a significant shift following the widespread implementation of vaccinations. While the introduction of the Haemophilus influenzae type b (Hib) vaccine led to a dramatic decline in cases of type b disease, there is an increasing prevalence of invasive infections caused by non-b and nontypeable strains of the bacteria.

Invasive disease is defined as the occurrence of bacteria in parts of the body that are normally free from germs, such as the blood. This is distinct from non-invasive Haemophilus influenzae infections, such as ear infections, which are not tracked by the Centers for Disease Control and Prevention (CDC).

The Impact of Hib Vaccination

Historically, Haemophilus influenzae serotype b was the primary cause of bacterial meningitis among children under five years of age. It was also a frequent cause of other serious invasive infections in young children, including pneumonia, bacteremia, and soft-tissue and skeletal infections.

The public health approach to these infections changed with the introduction of conjugate Hib vaccines. Vaccination began for children in 1987 and was expanded to infants in 1990. According to CDC and pediatric data, these measures resulted in a decrease of more than 99% in the annual incidence of invasive type b disease in children younger than five years old.

Rise of Non-b and Nontypeable Strains

As the incidence of type b disease remained low, other strains of the bacteria began to emerge as more prominent causes of invasive disease. Specifically, there has been an increase in infections caused by nontypeable bacteria and non-b types, primarily types a and f.

Data from the Active Bacterial Core surveillance system indicates that the incidence of disease caused by non-b and nontypeable bacteria increased until 2020. Following a period of fluctuation, these cases began increasing again in 2022.

High-Risk Populations

Surveillance data from 2019 through 2023 shows that the incidence of invasive disease remains highest among specific age groups. The populations most affected include infants younger than one year and older adults aged 65 years and older.

Nontypeable bacteria are particularly prevalent among these high-risk demographics, contributing to the ongoing challenge of managing invasive Haemophilus influenzae infections despite the success of the Hib vaccine.

Disease Surveillance and Tracking

To monitor these trends, the CDC utilizes two primary surveillance systems. Invasive Haemophilus influenzae disease is classified as a nationally notifiable disease, with information collected weekly through the National Notifiable Diseases Surveillance System (NNDSS).

the CDC gathers data from laboratories across 10 different areas of the United States through Active Bacterial Core (ABCs) surveillance. This system is part of the Emerging Infections Program and allows for the analysis and visualization of specific strain data to better understand the evolving nature of the bacteria.